The present disclosure relates to Positron Emission Tomography (PET) data acquisition, more particularly to effecting continuous bed motion (CBM) data acquisition in clinical time of flight (TOF) tomography.
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images which show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions which emanate from the body and are captured by a scintillation crystal, with which the photons interact to produce flashes of light or “events.” Events are detected by an array of photodetectors, such as photomultiplier tubes, and their spatial locations or positions are calculated and stored. In this way, an image of the organ or tissue under study is created from detection of the distribution of the radioisotopes in the body.
One particular nuclear medicine imaging technique is known as Positron Emission Tomography, or PET. PET is used to produce images for diagnosing the biochemistry or physiology of a specific organ, tumor or other metabolically active site. Measurement of the tissue concentration of a positron emitting radionuclide is based on coincidence detection of the two gamma photons arising from positron annihilation. When a positron is annihilated by an electron, two 511 keV gamma photons are simultaneously produced and travel in approximately opposite directions. Gamma photons produced by an annihilation event can be detected by a pair of oppositely disposed radiation detectors capable of producing a signal in response to the interaction of the gamma photons with a scintillation crystal. Annihilation events are typically identified by a time coincidence between the detection of the two 511 keV gamma photons in the two oppositely disposed detectors, i.e., the gamma photon emissions are detected virtually simultaneously by each detector. When two oppositely disposed gamma photons each strike an oppositely disposed detector to produce a time coincidence event, they also identify a line of response, or LOR, along which the annihilation event has occurred.
Time-of-flight positron emission tomography (TOF-PET) is based on the measurement of the difference between the detection times of the two gamma photons arising from the positron annihilation event. This measurement allows the annihilation event to be localized along the LOR with a adequate resolution.
Horizontal motion of the patient handling system (bed) traversing through the PET detector array is often needed during a single patient session for obtaining an elongated whole-body study. Such study requires acquisition of data to generate a single three dimensional image that typically extends from the patient's neck on through the pelvis area or further. The length of the subject image exceeds the physical extent of the axis of the stationary field of view (FOV) formed by the PET detector array. Traditionally, a “step and shoot” mode of whole body scanning is used for this purpose. The patient bed is moved to several fixed horizontal positions during the acquisition. These positions are typically overlapping and distributed along the patient thorax, with a separate static acquisition for each position. As each stationary acquisition completes, the bed will quickly move the patient to the next fixed position to service the start of the next step acquisition. The resulting static acquisitions may be separately reconstructed, with each image representing only the axial length of the stationary FOV. The resulting three dimensional images may be joined via voxel-summation in the overlapping regions to form a single assembled image. This single image represents an elongated region of the patient, typically with an axial length of 80 to 100 cm or more.
Techniques are needed to support effective data collection for a scanning mode in which the patient bed may be moved horizontally on a largely continuous basis, essentially without stopping, throughout one uninterrupted period of data acquisition. Continuous bed motion (CBM) would eliminate the “step and shoot” mode acquisitions that involve brief periods of rapid slewing movements between overlapping stationary bed positions in which acquisition of PET data is disabled.
These techniques should support effective CBM data collection for TOF, TOF-Mashing, and precise on-line mapping (rebinning) of physical LOR locations along the FOV axis into the projection data space. Data collection must be modified in real time with the horizontal movement of the patient bed. “Mashing” is a convenient short-hand expression in common use which refers to less precise transaxial angular sampling in the projection data space. TOF mashing enables acquisition of projection data sets that take up less memory space while preserving image resolution.